Urban Heat Mitigation Research Articles

A scaling law for predicting urban trees canopy cooling efficiency

Urban heat mitigation is a pressing concern for cities. Intense urban heat poses a threat to human health and urban sustainability. Tree planting is one of the most widely employed nature-based heat mitigation methods worldwide. Therefore, city policy makers require knowledge of how much temperature will be reduced by increasing urban tree canopy (UTC). Cooling efficiency (CE), which was been proposed to quantify the magnitude of temperature reduction associated with a 1% increase in UTC, has been primarily investigated at smaller scales previously. However, such small-scale results cannot be used to develop policy at the whole-city scale. This study developed a method that reveals the scaling relations of CE so as to predict its effects at the city scale. CE was found to follow the form of a power law as spatial scale increased from the small analytical units through intermediate size units up to the extent of a whole city. The power law form appeared consistently across cities with different climate backgrounds during summer daylight hours. Furthermore, the power law form was robust within cities under different summer weather conditions. The power-law scaling approach can thus be used to predict CE at the whole-city scale, providing a useful tool for managers to set UTC goals to mitigate extreme urban heat.

Integrative Remote Sensing Approaches Using Generative Adversarial Networks for Urban Heat Island Analysis and Mitigation

Integrative Remote Sensing Approaches Using Generative Adversarial Networks for Urban Heat Island Analysis and Mitigation

Radiometric characterization of daytime luminescent materials directly under the solar illumination

The materials efficiently emitting photoluminescence (PL) under solar irradiation could find broad applications in passive radiative cooling of buildings, urban heat mitigation, and solar energy harvesting. In this work, we discuss the limitations of common laboratory characterization of such materials (using tunable light sources or solar simulators) and propose a methodology for direct outdoor characterization of PL efficiency under full solar irradiation. A simple, portable radiometry setup with an integrating sphere is described, and its capability of rapid determination of the spectral distribution of the absorbed and emitted power, as well as the overall PL power efficiency and quantum yield, is demonstrated. By characterization of three materials developed for daytime radiative cooling applications, we reveal deviations of obtained parameters caused by the replacement of the real solar irradiation by that of solar simulators. The described method is suitable for studies of long-term stability, photo-induced degradation, or thermal effects.

Scale effect on the relationship between urban landscape patterns and land surface temperature

Great efforts have been made to examine the linkages between land surface temperature (LST) and urban landscape patterns (ULPs), which, however, focus on a single spatial scale and linear relationships. This study aims to examine the influence range of four key ULP indicators on LST, namely sky view factor (SVF) and three indices of built-up area, greens, and blue spaces. The analyses are conducted at street block level in 38 big cities in China, and leverage a multiscalar approach to investigate change patterns of indicators within multiple buffer zones and identify the buffer distance that yields the greatest influence. Results reveal that (1) the greatest local impacts are produced within 0−150 m buffer zones in most cities for all key ULP metrics; (2) the maximum impacts of most indicators in summer are greater than other seasons; and (3) the influence magnitude of key ULP indicators increase after considering scale effect, and the influence of SVF varies significantly across cities. Results suggest that the consideration of maximum influence ranges of ULP indicators can better explain LST spatial variations. Our findings offer evidence on the local impact of ULPs on LST and contribute to urban design in urban heat mitigation.

A WRF-UCM-SOLWEIG framework for mapping thermal comfort and quantifying urban climate drivers: Advancing spatial and temporal resolutions at city scale

The capability to evaluate city-scale outdoor thermal comfort is crucial for understanding the public's experience of heat stress, especially during heat waves. Nevertheless, there remains a methodological gap in accurately mapping thermal comfort with high spatial and temporal resolutions in complex urban environment and in quantifying the contributions of various urban climate drivers, such as sky view factors and tree canopy fraction. To bridge this gap, we propose an innovative WRF-UCM-SOLWEIG framework that couples a mesoscale model with an urban microclimate model. Specifically, it integrates the Weather Research and Forecasting model coupled with the urban canopy model (WRF-UCM) and the Solar and Longwave Environmental Irradiance Geometry model (SOLWEIG). This framework is then applied to a densely populated tropical city in China, to quantify the impact of urban climate drivers on the Universal Thermal Climate Index (UTCI). The results reveal a significant diurnal variation in the impact of urban morphology. Notably, the most significant drivers affecting daytime UTCI are the impervious surface fraction and the tree canopy fraction, with maximum Pearson's correlation coefficients of 0.80 and -0.70, respectively. These findings suggest that urban heat mitigation strategies should prioritize on reducing impervious surfaces and enhancing shade management, alongside expanding urban forestry measures.

Surface and canopy urban heat island disparities across 2064 urban clusters in China

The vast majority of urban heat island (UHI) studies are now derived from surface temperatures, substituting for the original air temperature-based definition. The disparities in hourly surface-canopy UHI effects (SUHI, CUHI) and the contrasting mechanisms are currently poorly understood. Here, we use high-resolution hourly LST and air temperature data from 2064 urban clusters in China to estimate SUHI and CUHI intensities and their driving mechanisms during the summer and winter of 2022. Across all urban clusters, we find that SUHI is on average ten times higher than the CUHI in the summer (0.97 VS. 0.09 °C) yet nearly triple that in the winter (0.30 VS. 0.11 °C), with SUHI exceeding CUHI in 91.5 % and 65.7 % of urban clusters, respectively. Seasonal and hourly analyses on SUHI/CUHI confirm typically opposite hysteresis variations (magnitude, peak, and timing of occurrence) and more correlated surface-canopy UHIs patterns during the night. We further demonstrate that SUHI magnitude can be largely explained by biophysical factors, urban attributes, and climate contexts, whereas CUHI interferes with additional constraints linked to ground-air energy transfer and advective dissipation. The improvement of urban greenery aids summer cooling efficiently in equatorial and boreal regions, while albedo measures are relevant in mitigating nocturnal warming in arid regions. Our findings support multiple technologies as ideas for urban three-dimensional UHIs (surface, canopy and boundary) and energy mechanisms, and the urgent need for ambitious urban heat mitigation strategies to minimize future climate change impacts.

Simulating the impact of ventilation corridors for cooling air temperature in local climate zone scheme

Climate change and urbanization are impacting urban microclimates, necessitating careful consideration in urban ventilation planning. However, there remains uncertainty regarding the identification and examination of urban ventilation corridors (VCs), and their mechanisms for urban heat mitigation. This study combined ENVI-met and the least-cost path (LCP) algorithm to simulate the VCs within the local climate zone (LCZ) scheme, with Guangzhou as an example. The results show that (1) VCs constructed using the LCP method are primarily distributed along vegetation and rivers, bypassing high-temperature aggregation in high-density/high-rise building areas; (2) The cooling effect of VCs varies across different LCZs, influenced by factors such as land use, vegetation, building, and topography. (3) In low-density, low-rise LCZs, VCs significantly reduce temperatures by about 0.23°C, while in high-density, high-rise LCZs, the cooling effect is weaker, with a reduction of only around 0.01°C. This study highlights the importance of preserving hydrological system and optimizing green space layout to enhance urban ventilation thus mitigate heat accumulation.

Micro-Urban Heatmapping: A Multi-Modal and Multi-Temporal Data Collection Framework

Monitoring microclimate variables within cities with high resolution and accuracy is crucial for enhancing urban resilience to climate change. Assessing intra-urban characteristics is essential for ensuring satisfactory living standards. This paper presents a comprehensive methodology for studying urban heat islands (UHIs) on a university campus, emphasizing the importance of multi-modal and multi-temporal data collection. The methodology integrates mobile surveys, stationary sensor networks, and drone-based thermal imaging, providing a detailed analysis of temperature variations within urban microenvironments. The preliminary findings confirm the presence of a UHI on the campus and identify several hotspots. This comprehensive approach enhances the accuracy and reliability of UHI assessments, offering a cost-effective, fine-resolution approach that facilitates more effective urban planning and heat mitigation strategies.

Measuring and monitoring tree cover and plant canopy height in Pune city, India

Global datasets often fall short in estimating tree cover areas outside the forest tracts, particularly in urban areas. This limitation poses a challenge for accurately estimating tree cover outside forests (ToF) and understanding their contribution to ecosystem services. In the context of increasing population pressure and unsustainable urbanization in Indian cities, regional-level tree cover and canopy height estimations are crucial for evaluating the impact of ToF on various ecosystem services, including carbon sequestration, climate change adaptation, biodiversity hotspots, groundwater recharge, soil conservation, flood control, urban heat mitigation, and psychological and emotional well-being. This technical note outlines a rapid and cost-effective methodology for assessing tree canopy cover and height, specifically tailored for ToF estimation, offering a valuable tool for addressing these socio-urban environmental challenges and enabling sustainable land use planning.

Prioritizing social vulnerability in urban heat mitigation.

We utilized city-scale simulations to quantitatively compare the diverse urban overheating mitigation strategies, specifically tied to social vulnerability and their cooling efficacies during heatwaves. We enhanced the Weather Research and Forecasting model to encompass the urban tree effect and calculate the Universal Thermal Climate Index for assessing thermal comfort. Taking Houston, Texas, and United States as an example, the study reveals that equitably mitigating urban overheat is achievable by considering the city's demographic composition and physical structure. The study results show that while urban trees may yield less cooling impact (0.27 K of Universal Thermal Climate Index in daytime) relative to cool roofs (0.30 K), the urban trees strategy can emerge as an effective approach for enhancing community resilience in heat stress-related outcomes. Social vulnerability-based heat mitigation was reviewed as vulnerability-weighted daily cumulative heat stress change. The results underscore: (i) importance of considering the community resilience when evaluating heat mitigation impact and (ii) the need to assess planting spaces for urban trees, rooftop areas, and neighborhood vulnerability when designing community-oriented urban overheating mitigation strategies.

Dwelling conversion and energy retrofit modify building anthropogenic heat emission under past and future climates: A case study of London terraced houses

Energy demand per capita is expected to increase with smaller dwelling units in Europe, as less energy is shared linked to the trend of fewer people per household. To meet the demand for more smaller units, a popular retrofitting approach is to split existing large dwellings. As this type of dwelling conversion (DC) affects both household size (HHS) and therefore the likely energy use behaviour of residents, building thermal performance and anthropogenic heat emission (QF,B) to outdoor environment are impacted. Here, the UK time use survey (TUS) provides activity information to allow comparison of the implications of DC to energy conservation measures (ECM) for terraced houses in both past and future London climates. Our results show that ECM can substantially reduce both heating energy demand and QF,B during cold seasons, whilst due to the absence of space cooling in UK residential buildings the ECM ineffectively diminishes summer demands. Further to this, the increased occupancy density resulting from DC increases summer peak QF,B by 53.8% at 17:00, which could intensify canopy-layer urban heat island effects. Although climate projected for the 2050′s should result in a decreased wintertime QF,B, the potential increase in summertime space cooling energy demand will see an associated increase in summertime QF,B. Occupancy patterns need to be considered as part of retrofitting assessments and climate change impacts due to their influence on HVAC usage schedule. The role of occupancy behaviour extends beyond retrofit strategies themselves, to larger urban extents (e.g. planning, policy making, urban weather/climate feedbacks) to ensure both energy saving and urban heat mitigation.

Evaluating the Impact of Green Spaces on Urban Heat Reduction in Rajshahi, Bangladesh Using the InVEST Model

Urban heat poses significant challenges in rapidly developing cities, particularly in countries like Bangladesh. This study investigates the cooling effects of urban green spaces in Rajshahi city, addressing a critical research gap in developing urban contexts. We examined the relationships among urban vegetation, heat mitigation, and temperature variables using the InVEST Urban Cooling Model and spatial analysis techniques. This study focused on three key relationships: Normalized Difference Vegetation Index (NDVI) and Heat Mitigation Index (HMI), HMI and Land Sur face Temperature (LST), and HMI and Air Temperature (AT). Analysis revealed a strong positive correlation between NDVI and HMI, indicating the effectiveness of vegetation in enhancing urban cooling. A robust inverse relationship between HMI and LST was observed (R2 = 0.78, r = −0.88), with every 0.1 unit increase in HMI corresponding to a 0.53 °C decrease in LST. The HMI−AT relationship showed an even stronger correlation (R2 = 0.84, r = −0.87), with each unit increase in HMI associated with a 2.80 °C decrease in air temperature. These findings quantify the significant role of urban green spaces in mitigating heat and provide valuable insights for urban planning in developing cities, underscoring the importance of integrating green infrastructure into urban-development strategies to combat urban heat and improve livability.

Where is the heat threat in a city? Different perspectives on people-oriented and remote sensing methods: The case of Prague

Extreme heat in urban areas has a severe impact on urban populations worldwide. In light of the threats posed by climate change, it is clear that more holistic and people-oriented approaches to reducing heat stress in urban areas are needed. From this perspective we aim to identify and compare thermal hotspots and places with favourable thermal conditions, based on three different methods – thermal walk, participatory-based cognitive mapping, and remote sensing in a Central European city. Although major hotspots in large low-rise development zones were identified by all three methods, the overall agreement between on-site thermal sensation votes, cognitive maps and surface temperatures is low. In the urban canyon of compact mid-rise and open mid-rise development, the thermal walk method proved to be useful in the identification of the specific (parts of) streets and public spaces where citizens can expect thermal discomfort and experience heat stress, e.g. crossroads, arterial streets with a lack of greenery, north facing unshaded parts of streets, and streets with inappropriate tree spacing. Cognitive maps on an urban neighbourhood scale are not specific enough on a street level; however, as a supplementary method they can help identify discrepancies between on-site sensations and thermal conditions. For further research on effective and cost-efficient urban heat mitigation, we suggest combining thermal walks with numerical model simulations.

Vertical thermal environment investigation in different urban zones (LCZ4/LCZ6/LCZA) and heat mitigation evaluation: Field measurements and numerical simulations

Urban heat island (UHI) effect has become a main urban environmental problem in megacities. To express the UHI effects more meaningfully and provide scientific basis for heat mitigation and urban planning, both field measurements and numerical simulations were conducted in Xi'an City, China. Three typical urban local climate zones (LCZs) with close geographical locations—high-rise/low-rise residential areas (LCZ4/6) and urban parks (LCZA)—were chosen to investigate thermal environments in vertical spaces. It was found that UHILCZ6/4−LCZA could reach 1.2/2 °C during the daytime and 0.3/0.7 °C in the night at screen-level height. In particular, temperature inversions were found in residential areas during the daytime, which led to an increase in the UHI intensity at certain heights. To investigate means to improve the thermal environment of residential areas, numerical simulations were carried out to test the influence of cool roofs and cool facades. It was found that the daytime air temperatures could be reduced by 0.2 °C and 0.4 °C after adopting high-reflectance roofs and facades in high-rise residential area at screen-level height, respectively. In addition, the implementation of cool facades also affected the air temperature profiles and atmospheric conditions, leading to the disappearance of temperature inversions and complicated stratification, and providing more evident cooling effects at certain heights. The findings of this study provide new insights into thermal environments of urban zones with different structures and have practical importance for urban UHI mitigation.

Evaluating cooling effect of blue-green infrastructure on urban thermal environment in a metropolitan city: Using geospatial and machine learning techniques

Urban blue-green infrastructures (BGIs) have a considerable cooling effect, reducing the urban heat island effect. Therefore, this study aims to evaluate the cooling effects of BGIs and role of the influencing factors on urban thermal environment in Kolkata city, which is one of the largest mega cities of India. An integrated radius and non-linear regression approach was used to determine the maximum distance over which cooling is effective. Further, the relationship between cooling effects of BGIs and influencing factors was analysed using Random Forest based Explainable artificial intelligence model. The result shows that for most of the BGIs, the range of maximum cooling distance varies from 210 to 240 m from the boundary of BGIs. The average PCA value ranges from 6.12 to 16.77 hectares and PCE value ranges from 1.62 to 2.87 depending of BGIs. The results of PCA, PCE and maximum cooling distance indicate that the spatial configurations of BGIs and landscape pattern within and surrounding of BGIs sites strongly influence the cooling effects. The outcomes of this study provide valuable insights for the policymakers and urban planner about the intervention of blue-green infrastructures for the better urban environmental conditions and heat mitigation in mega cities and other urban areas.

Long-term temporal and spatial divergence patterns of urban heat risk in the Beijing–Tianjin–Hebei urban agglomeration

As urbanization accelerates and global warming intensifies, urban heat risks are increasingly becoming a critical issue and addressing this challenge to foster more harmonious and livable urban environments is essential. In our study, multisource remote sensing and point-of-interest (POI) data were applied to assess the urban heat risks in the Beijing-Tianjin-Hebei (BTH) urban agglomeration from 2001 to 2020. The results indicated that the heat hazard, exposure, and sensitivity were notably greater in the southeastern of the BTH. Conversely, the heat adaptability was lower in the southeast, leading to a severe high heat risk in this region. From 2001 to 2020, the heat risk index (HRI) values for these cities generally displayed an upward trend, peaking in 2015. Among the cities, Handan had the highest heat risk, followed by Baoding and Beijing, with HRIs of 0.32, 0.32, and 0.31, respectively. In urban areas, commercial zones had the highest average HRI at 0.3, while public management zones had the lowest at 0.28. These findings highlight significant variations in heat risk, indicating the need for targeted urban planning and heat mitigation strategies in the most affected areas of the BTH.

A comprehensive metric scheme for characterizing the heterogeneity of urban thermal landscapes: A case study of 14-year evaluation in Beijing

Urbanization has affected land surface temperature (LST) significantly. The spatial average of LST is widely applied to evaluate urban heat islands (UHI), but a simple comparison of mean temperature is insufficient to evaluate the heterogeneity of LST and the effectiveness of urban heat mitigation strategies. This study took the spatial distribution of LST as the thermal landscape and firstly employed 3D landscape metrics to analyze its spatial heterogeneity. To perform collaborative analysis with UHI, the urban–rural differences of landscape metrics were defined as thermal metric islands (TMI). Significant UHI and TMI were revealed by temperature amplitude, aggregation, and complexity parameters, indicating higher temperature fluctuation and fragmentation in urban regions. Before 2007, the standard deviation of thermal surface, landscape fragmentation, and shape irregularity increased significantly, particularly in the suburbs. Later, benefiting from the Summer Olympic Games, the temperature fluctuation, fragmentation, and shape complexity decreased obviously, and UHI intensity hit the lowest. Both UHI intensities and TMI increased after 2010, and various metrics suggest that maximum temperatures, landscape fragmentation, and shape complexity declined over most of the district, even as the mean temperatures increased. This study concludes that urban temperatures have risen at a higher rate than suburban areas, but the thermal landscape composition and configuration have changed more dramatically in rural areas.

A Systematic Assessment of Greening Interventions for Developing Best Practices for Urban Heat Mitigation—The Case of Huế, Vietnam

The health of urban populations is increasingly at risk due to the amplification and chronification of urban heat stress by climate change. This is particularly true for urban environments in humid tropical climates, including many cities in Southeast Asia. It is also in these locations where increasing climatic risks may be exacerbated by urban growth, underscoring the need to develop effective mitigation strategies for strengthening urban resilience and supporting climate change adaptation. Conservation and widespread implementation of green infrastructure (GI) are regarded as one means to counter heat as a public health threat. However, for lower-income countries across Southeast Asia, such as Vietnam, knowledge gaps remain with respect to the effectiveness of greening interventions for heat mitigation. To address this gap, in the context of urban expansion in the humid tropical city of Huế, Vietnam, diurnal cooling potential and regulation of outdoor thermal comfort (OTC) within a wide, shallow street canyon were systematically assessed for selected elements of GI along a quantitative and qualitative dimension using ENVI-met. Tree-based interventions were found to be most effective, potentially decreasing UTCI by −1.9 K at the domain level. Although lower in magnitude, green verges and green facades were also found to contribute to OTC, with green verges decreasing UTCI by up to −1.7 K and green facades by up to −1.4 K locally. Potential synergistic cooling impacts were identified through a combination of GI elements. However, no scenario was found to decrease heat stress to zero or moderate levels. Substantially reducing heat stress may thus require further measures and a closer consideration of local morphological characteristics.

Evaluating the Urban Heat Mitigation Potential of a Living Wall in Milan: One Year of Microclimate Monitoring

Urban heat island (UHI) mitigation and adaptation are urgent needs in a built environment, which requires us to search for sustainable solutions to limit the urban heat island effect and improve the energy efficiency of building envelopes. Among these solutions, vertical green structures (VGSs) have recently attracted significant attention for their potential to mitigate adverse effects, especially in densely built areas. This study presents a comprehensive data analysis of the microclimate of a living wall in Milan, Italy. Our aim was to evaluate this VGS’s performance in mitigating temperature increases caused by the UHI effect. In the literature, similar studies are limited to shorter monitoring periods (mostly in cooling seasons) and specific orientations (mostly facing south). However, the VGS presented in this case study here faces northwest and was continuously monitored for one calendar year. During this continuous in situ monitoring campaign, air temperature data from sensors either embedded in vegetation or exposed on a bare wall were collected and analysed over a whole calendar year, which is a novelty compared to the existing literature focused on VGSs due to the long duration. The findings indicate that the studied VGS has the ability to influence the outdoor microclimate depending on the season, the precipitation events, the wall exposure, the type of vegetation, and the vegetation’s phenological attributes. The analysis showed that the VGS consistently maintained cooler temperatures than the bare wall, with mean temperature differences ranging from 2.8 °C in autumn to 0.8 °C in spring through the winter. The vegetation acted as a natural insulator by reducing the air temperature during the hot summer and in early autumn, corresponding to the growing period of the vegetation. Thus, VGSs show potential to mitigate the global warming effect. These findings provide valuable insights on vegetation’s capability to act as a thermal regulator for sustainable urban planning and energy-efficient building design and retrofitting.

Reviewing Study of the Urban Heat Island Phenomenon and Mitigation Strategies Using Available Technology

Reviewing Study of the Urban Heat Island Phenomenon and Mitigation Strategies Using Available Technology